How Humans See Colors - What Causes Color Blindness?

Color Blindness, as explained in earlier post, is the inability to identify or differentiate different colors. To understand the causes of color blindness, it is important to first learn how human eye sees or percieves color normally.

How We See Color

When light hits an object - lets say a red apple - some of the light is absorbed based on properties of the object and some is reflected into our eyes.

Special cells in our eyes detect the wavelength of reflected light and our brain processes the information so we percieve the color of object - as Red - based on that wavelength. What we see as color - red - is actually the wavelength of that reflected light.

Visible light, that human eye can process, is made up of a spectrum of wavelengths each corresponding to a different color.

Light Sensitive Cells In Human Eyes

Special cells in our eyes that can detect wavelengths of light and enable us to differentiate between colors. They are of two types called Rods and Cones.

Rods are only active in low or dim light, they enable us to see shades of black and white in low light conditions.

Cones are activated in bright light and are sensitive to wavelength of light corresponding to red, green or blue colors.

Cones are the light sensitive cells that enable us to see different colors. Cone cells are of three types based sensitivity to Red, Green and Blue light. Red Cones activate when they detect light on spectrum of wavelenght corresponding to Red color. Similarly Green and Blue Cone Cells will sense Green and Blue light. All the visible colors are a combination of Red, Blue and Green light. So when Cone cells detect Red and Green reflected light our brain percieves it as Brown color, which is a combination of Red and Green.

Our brain can detect different wavelengths of light using red, green and blue sensitive cone cells in eyes and percieve colors by combining these primary light colors.

What Causes Color Blindness

Our ability to see and differentiate colors depends on these light sensitives cells, Rods and Cones, and if these cells don't function properly that ability to see color is lost as in case of Color Blindness.

Color Blindness occurs when one or more of these Light sensitive cells are not fuctioning properly or are completely absent. If red cone cells are absent then human eye won't be able to detect any color made with combination of Red with green or blue colors.

In Different types of color blindness, one or more of these cones are absent or damaged causing color vision deficiency.

Normal vision people will see a number 74 in the image above, most people with mild color blindness might see 21 and those with strong color blindness won't be able to see any number at all. This is because some of the cone cells responsible for detecting that color might not be functioning for color blinds making it hard to differentiate between different colors.